1. Field of the Invention
This invention relates to a thin film transistor (TFT) having an insulating film and to a display having such a TFT as a switching element.
2. Description of the Prior Art
In recent years TFTs having a poly silicon film as an active layer have been developed as a driver element or a pixel driving element for use in various displays, such as an active-matrix liquid crystal display (LCD).
An LCD having a conventional TFT will be described below.
FIG. 1 is a plan view of a conventional TFT used in a display pixel section; FIG. 2 is a cross-sectional view along E--E line in FIG. 1 of an LCD using such a TFT.
As shown in FIG. 1, the TFT in the pixel section is formed near the intersection of gate signal line G for providing gate signals and drain signal line D for providing image signals, and its source is connected to a display electrode.
The structure of such a TFT will be described with reference to FIG. 2.
A gate electrode 2 made of a refractory metal (high melting point metal), such as chromium (Cr) and molybdenum (Mo), a gate insulating film 3, and an active layer 4 made of a poly silicon film are formed in that order on an insulating substrate 1 made of quartz glass, non-alkaline glass, or the like.
In the active layer 4, a channel 7 is formed over the gate electrode 2 and a source 5 and a drain 6 are formed with the channel 7 between by ion doping with a stopper insulating film 8 on the channel 7 as a mask.
An interlayer insulating film 9 consisting of an SiO.sub.2 film, an SiN film, and an SiO.sub.2 film layered is formed over the whole of the gate insulating film 3, active layer 4, and stopper insulating film 8. Furthermore, a drain electrode 10 is formed by filling a contact hole made in the interlayer insulating film 9 at a position corresponding to the drain 6, with a metal, such as Al. In addition, a planarization film 12 made of, for example, an organic resin for planarization the surface is formed entirely over the interlayer insulating film 9. A contact hole, at a position corresponding to the source 5, is made in the planarization film 12. Furthermore, a display electrode 13, which is a transparent electrode and also serves as a source electrode, made of ITO (indium tin oxide) being in contact with the source 5 via this contact hole is formed on the planarization film 12. An alignment film 14 made of an organic resin, such as polyimide, for aligning liquid crystal 24 is formed over the display electrode 13.
The insulating substrate 1 on which the TFT is formed, and an opposite substrate 20 having an opposite electrode 21 and an alignment film 22 opposite to the insulating substrate 1, are bonded together at their edges with a sealing adhesive 23 and a vacant space formed is filled with liquid crystal 24.
In the structure of such a conventional TFT, however, impurities or impurity ions generated when the sealing adhesive hardens, water or impurity ions included in the liquid crystal 24, water permeating from the outside through a gap 25 occurring when the sealing adhesive 23 comes off, water included in the air coming in contact with the planarization film 12 (alignment film 14 for a display portion in an LCD), or other substances become attached to the surface of the planarization film 12, so that electric charge occurs on the surface of the planarization film 12 and polarization occurs between the top and the bottom of the planarization film 12 or each film of the interlayer insulating film 9.
Therefore, a back channel is formed in a TFT, resulting in the disadvantage of a change in a threshold voltage of the TFT.
Furthermore, if such a TFT is used in an LCD, an increase in a threshold voltage of the TFT leads to a decrease in an ON-state current of the TFT and, conversely, a decrease in a threshold voltage of the TFT leads to an increase in an OFF-state current of the TFT. If such a TFT is used as a pixel switching element in, for example, a normally-white mode LCD, a bright point defect, in which some pixels remain permanantly bright, may occur in these cases. As a result, image display is of poor quality. In addition, if a threshold voltage varies from TFT to TFT, there exists the disadvantage of being unable to obtain display having uniform brightness all over.